Senators investigating President Donald Trump’s ties to Russia wanted to show they meant business when they slapped Michael Flynn with a subpoena last week — but the aggressive move may be less than it seemed.

When an obstinate subject decides to dig in, congressional subpoenas often don’t succeed. And when lawmakers vote to hold someone in contempt of Congress, the cases frequently end up in court, sometimes stalled for years. Other times, members of Congress have avoided a showdown altogether, recognizing the limits of their authority.

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Republicans, for instance, never secured testimony from Hillary Clinton’s former technology aide, Bryan Pagliano, about her private email set-up, despite a subpoena and a committee vote last year to hold him in contempt. In 2012, the Justice Department declined to prosecute former Attorney General Eric Holder after he defied a congressional subpoena, citing executive privilege. And Justice declined to go after former IRS official Lois Lerner for contempt in 2015, even though she had refused to testify before a House panel.

The drawn-out congressional subpoena process underscores the limitations of the congressional investigations into Trump’s Russia ties — and why the FBI’s investigation, which Democrats fear could be in jeopardy after Trump’s firing of James Comey, is the one that really matters.

The House and Senate Intelligence committees are set up to oversee the investigative branch, not conduct sprawling investigations into potentially criminal acts. And they are largely dependent on the FBI, CIA and other federal agencies to provide them with information.

And when they are stonewalled by a witness, there's often little the committees can do to force quick compliance. Congressional subpoenas, according to lawyer Stanley Brand, sometimes amount to little more than “a piece of paper.”

“The problem is enforcing them,” said Brand, a senior counsel at Akin Gump who has represented individuals under investigation by Congress. “They can go the civil route in the Senate, but that’s a lengthy process. That can take a year or more. They could go under the criminal statute, but that’s sort of unavailing because by the time that gets decided, it’ll be the next Congress.”

The subpoena process could be a key test of Senate Republican leaders’ commitment to getting to the bottom of Moscow’s involvement in the 2016 presidential election and the possibility of collusion with the Trump campaign.

If Flynn refuses to provide the requested documents — and a March statement from his lawyer suggests he might not cooperate without “assurances against unfair prosecution” — then Senate leaders would have to decide whether to vote to hold him in contempt. The matter would then go either to the Justice Department, which could file criminal charges, or to federal district court for civil enforcement.

The decision to vote to hold Flynn in contempt could be a tough one for Senate Majority Leader Mitch McConnell (R-Ky.), who has so far backed Trump at every turn of the Russia saga. The issue would force him to choose between his “political prerogatives” of protecting a Republican president and his “institutional prerogatives” of safeguarding the authority of the Senate, according to a Democratic congressional aide who has experience with subpoenas.

“I would think some members might, even if it’s tough for them politically, see that they need to uphold the rights of the body,” said the aide, who spoke on the condition of anonymity. “If you have a person defying the body, are you going to let them get away with that?”

The House and Senate Intelligence Committees are investigating Trump campaign aides over allegations of coordination with Russia, as is the FBI.

The Senate probe, led by Intelligence Chairman Richard Burr (R-N.C.), has been under criticism from Democrats, who say it’s moving too slow. Burr answered those charges last week with the decision to subpoena Flynn, a move that sources said was designed to send a message to other people in Trump’s orbit that the committee is willing to be aggressive with those who won’t provide documents voluntarily.

Flynn’s lawyer, Robert Kelner, declined to comment on whether his client would comply with the subpoena. Flynn likely could avoid testifying before the intelligence panel by asserting his Fifth Amendment rights against self-incrimination, but this might not apply when it comes to a subpoena for existing documents.

Some other witnesses are complying with the committee’s requests for documents, while others are not, sources said. One witness who is not complying is former Trump foreign policy adviser Carter Page, though Burr indicated last week he was not quite ready yet to subpoena Page.

“I think in some cases we have seen — in the case of Carter Page publicly — a complete reversal in his willingness,” Burr said. “You wait another day, and he may be back to voluntarily doing it. So I don't want to rush to judgment. I'd rather wait until we see firmly where people are.”

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The right of Congress to compel witness testimony is not listed explicitly in the Constitution but has been recognized since the 1790s, when men accused of attempting to bribe a House member were arrested by the sergeant-at-arms and hauled before the House for trial, according to the Congressional Research Service.

Congress, though, has not used its power to arrest and detain witnesses since 1935, instead referring contempt cases to the Justice Department or civil courts.

Witnesses facing subpoenas often reach a deal with Congress to limit the scope of the request for documents. In the case of witnesses who refuse to comply, Congress sometimes does succeed in forcing them to turn over documents — but it’s a difficult process that can take years.

“The criminal contempt process takes forever — you vote [for] it and then it goes to the U.S. attorney and he has to decide what to do with it,” Brand said. “If they go the civil route, it takes just as long. No matter what option they take, it’s not quick.”